def close_union(self, nfa: NFA) -> None:
"""
close_union
Close the last open union by connecting the two halves
(from self.union_in_progress and self.last_state) to a new state via epsilon
"""
logging.debug('Closing union: ' + str(self.union_in_progress[-1]))
# connect last state of each branch
self.last_state = nfa.close_branch(self.last_state,
self.union_in_progress[-1][1])
self.union_in_progress = self.union_in_progress[:-1]
